1. Field of the Invention
The present invention relates to a radiation imaging panel suitable for application to a radiation imaging apparatus using an X-ray and the like, and specifically to a method for manufacturing a photoconductive layer constituting the radiation imaging panel.
2. Description of the Related Art
Heretofore, in medical X-ray imaging, an X-ray imaging panel has been known, which uses a photoconductive layer sensitive to an X-ray as a photosensitive member for the purpose of reducing a dose of radiation exposed to a subject, improving diagnostic performance, and so on, reads an electrostatic latent image formed on the photoconductive layer by the X-ray by means of light or a large number of electrodes, and records the image thus read. A method using the X-ray imaging panel is superior in that resolution thereof is higher than fluorography by a television camera tube, which is a well-known imaging method.
The above-described X-ray imaging panel is constituted to generate charges corresponding to X-ray energy by irradiating a charge generation layer provided therein with the X-ray, and to read the generated charges as electric signals. The above-described photoconductive layer acts as the charge generation layer. Heretofore, amorphous selenium has been used for the photoconductive layer. However, X-ray absorptivity of amorphous selenium is low in general. Accordingly, it is necessary that the thickness of the photoconductive layer be formed thick (for example, 500 μm or more).
However, when such film thickness is thickened, reading speed of the latent image decreases. In addition, high voltage is applied to the photoconductive layer at least during a period from a start of the reading to an end thereof after the latent image is formed. Accordingly, dark current increases, and charges caused by the dark current are added to the charges of the latent image, which thereby decrease a contrast in a low-dose range. These are regarded as problems. Moreover, the high voltage is applied to the photoconductive layer, and accordingly, device deterioration is prone to occur, durability thereof is prone to decrease, and electric noise is prone to occur. Furthermore, the photoconductive layer is usually formed by an evaporation method, and accordingly, it takes a considerable time to grow the photoconductive layer until the photoconductive layer reaches the thickness as described above, and management of such a growth process is also cumbersome. These factors eventually lead to an increase in manufacturing cost of the photoconductive layer, which bring about an increase in cost of the X-ray imaging panel.
Considering the problems as described above, materials for the photoconductive layer other than the selenium are being studied. For example, in Japanese Unexamined Patent Publication Nos. 11 (1999)-237478 and 2000-249769, as a material constituting the photoconductive layer, a bismuth oxide-series complex oxides represented by a composition formula BixMOy (where M is at least one of Ge, Si and Ti, x is the number satisfying a condition 10≦×≦14, and y represents the stoichiometric atomic number of oxygen according to M and x which are described above) is described. In accordance with the bismuth oxide-series complex oxides, it can be expected that charge conversion efficiency of the X-ray will be improved.
In general, with regard to Bi12MO20 synthesized by a solid phase method which fires Bi2O3 and MO3 at 800° C., there is a problem that a collection effect of the generated charges is poor because a particle diameter thereof becomes as large as a size in a micron order and a filling density of the formed photoconductive layer is low. Incidentally, in the above-described Japanese Unexamined Patent Publication Nos. 11 (1999)-237478 and 2000-249769, the following is described as a method for forming the photoconductive layer. Specifically, sol or gel obtained by performing hydrolysis for alkoxides of the bismuth and the metal is subjected to a sintering treatment, and the sol or gel thus sintered is dispersed and coated, thereby forming the photoconductive layer (hereinafter, this method is referred to as a sol-gel method).
Moreover, in the above-described Japanese Unexamined Patent Publication Nos. 11 (1999)-237478 and 2000-249769, the sol or gel obtained by performing the hydrolysis is subjected to the sintering treatment, and rapid crystal growth occurs owing to the sintering treatment. Accordingly, the size of generated particles becomes large, and therefore, the filling density of the photoconductive layer cannot be raised to a satisfactory level. Moreover, the above-described Japanese Unexamined Patent Publication Nos. 11 (1999)-237478 and 2000-249769 only describe that the alkoxides of the bismuth and the metal are subjected to the hydrolysis, and specific liquid characteristics thereof and the like are unknown. In addition, if only the alkoxides are reacted with each other in a liquid phase, the alkoxides become amorphous particles without being crystallized. However, if the amorphous particles are left as they are, electron mobility thereof is low, and the amorphous particles have so many traps that electrons are captured thereby quickly, and accordingly, photoconductivity thereof is deteriorated.